= Intercept-resend attack
An eavesdropper measures quantum carriers and forwards states consistent with the outcomes. The resulting disturbance is tested statistically by the legitimate users. For a singlet measured by Eve on both sides along $\mathbf n_a,\mathbf n_b$, the forwarded correlation is $-(\mathbf a\cdot\mathbf n_a)(\mathbf n_a\cdot\mathbf n_b)(\mathbf b\cdot\mathbf n_b)$. It follows by averaging the product of the conditional local outcome means over Eve's correlated outcomes. This model does not cover every general quantum attack.
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